Electric Field Tunable Magnetism at C6H6-Adsorbed Fe3O4(001

Article pubs.acs.org/JPCC

Electric Field Tunable Magnetism at C6H6‑Adsorbed Fe3O4(001) Surface Meifang Sun,† Dongxing Zheng,† Xiaocha Wang,‡ and Wenbo Mi*,† †

Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparation Technology, School of Science, Tianjin University, Tianjin 300354, China ‡ School of Electronics Information Engineering, Tianjin University of Technology, Tianjin 300384, China ABSTRACT: The use of molecular modification on magnetism has gained considerable interest in the development of multifunctional molecular spintronics. Such hybrid structures of nonmagnetic molecules and ferromagnetic metals manifest great promises of producing novel electric and magnetic features. The external electric field effect on the magnetism of C6H6-adsorbed Fe3O4(001) surface is elucidates by density functional theory calculations. The reduced magnetic moments of partial octahedral Fe atoms in the first layer break the spherical spatial spin density distribution. Such modification that is independent of the direction of electric field can be attributed to the charge redistribution as a result of screening effect, which changes orbital occupancy in unpaired octahedral Fe-d electrons near EF accompanied by a spin flip. Furthermore, octahedral Fe atom underneath C atom changes only as the applied field is large enough. Additionally, it is shown that the study of modulation on surface magnetism through external electric field is expected to excite a new area in molecular spintronics, such as the potential applications in electrically controlled magnetic data storage.

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INTRODUCTION

Furthermore, the modification on the magnetic properties of nanomaterials is mainly focused on the multifunctionally multiferroic materials based on the magnetoelectric effect, where the polarization induced by external electric field changes the structure of materials and manipulates the magnetism of materials.21 By means of the modulation of external electric field, it is possible to achieve new data storage and processing devices with low energy consumption and high controllability.22−24 A large variety of experimental and theoretical studies have been paid, including electrically controlled ferromagnetic states,25,26 magnetic anisotropy,27 exchange bias,28 and surface/interface magnetization.29,30 The realization of advantages of electric field modulation in hybrid molecule/ metal structure will further develop the multifunctional molecular spintronic devices. For an instance, the magnetization direction of Fe−phthalocyanine molecule adsorbed on O−Cu(110) surface was found to be switchable by applying an external electric field along the surface normal.31 Furthermore, a common attention to study the interfacial behaviors between molecule and substrate is mainly based on the strongly hybridized ones, and the weak interaction strengths are rarely reported.32,33 So, this work will investigate the electric-field effect on magnetic properties of C6H6-adsorbed Fe3O4(001)

Molecular spintronics, the integration of molecular electronics with spintronics, have shown striking avenues in building multifunctional technological applications.1−4 From a scientific point of view, the molecular-based devices are appealing as a result of long spin relaxation time due to weak spin−orbit interaction.5 From a realistic point of view, the molecular structures and properties are convenient to manipulate by variation of functional groups. The adsorption of aromatic molecules at magnetic metal surface, as an essential part for the molecular spintronics and surface science studies, has attracted considerable attentions.6−9 Such organic/metallic interfacial systems present great promise as certified with the implementation of organic spin valves,10 magnetic tunnel junctions,11,12 molecular spin filtering,13,14 etc. What’s more, along with the technically realization of fabricating high-quality self-assembled monolayers of molecules on metal and oxide surfaces,15,16 the modulation on the magnetic properties of materials via adsorbing molecule becomes an available choice. For example, it has been experimentally found a effective manipulation of ferromagnetism in the (Ga,Mn)As films by depositing organic molecules on the surface.17 The perpendicular magnetic anisotropy of a Co (Fe) thin film is enhanced (decreased) after covering a C60 overlayer.18 Theoretical calculations also have revealed that the surface magnetism can be modified through adsorbing molecules.19,20 © XXXX American Chemical Society

Received: January 19, 2017 Revised: February 22, 2017 Published: February 22, 2017 A

DOI: 10.1021/acs.jpcc.7b00617 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C surface, where a weak interaction effect is expected to occurs due to the low reactive nature of Fe3O4 surface.32 Fe3O4 has a inverse spinel structure with Fe3+ ions at tetrahedral A sites (FeA) and Fe2+, Fe3+ cations at octahedral B sites (FeB). The superiorities of high Curie temperature (858 K) and halfmetallic character34 make it a potential candidate in spintronic devices. As a basic structure of most organic molecules, it is important to study the adsorption of C6H6 itself on the magnetic surface. It is found that the induced redistribution of electron orbital occupation results in the reduction of local atomic magnetism in the first layer of Fe3O4(001). The FeB directly bonded to C atom is dependent on the magnitude of field. These findings are interesting in the field of electrically controlled magnetism, which have the potential applications in molecular spintronic devices.

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Figure 1. (a) Cubic inverse spinel structure of bulk Fe3O4. (b) Top view of the first two layer for FeBO termination. (c) Top and (d) side view of FeB-top model.

COMPUTATIONAL DETAILS AND MODEL The calculations are utilized by a plane-wave basis set35 in the framework of density functional theory (DFT), as carried out with the Vienna ab initio simulation package.36 The exchangecorrelation function is described by Perdew-Burke-Ernzerh of generalized gradient approximation.37 The energy cutoff for the plane-wave expansion is 500 eV, and a 4 × 4 × 4 and 4 × 4 × 1 k-point set is employed to Brillouin zone integration for the bulk and adsorption system, respectively. The surface is modeled by a nine-layer Fe3O4 slab through a 1 × 1 unit cell with a vacuum region of 15 Å along the axis z. Since the distance between adsorbed C6H6 in the neighboring unit cells is about 8 Å, the spurious interactions between molecules should be considered. Thus, the simulation may be related to the case of a relatively high coverage. The DFT-D2 method38 is applied to describe the van der Waals (vdW) interactions since it provides a computationally efficient and numerically accurate description of the interaction between molecule and surface.39−41 The bottom four layers of substrate are constrained, while the atomic positions in other layers and molecule are fully allowed to relax until all residual forces are less than 0.03 eV/Å as well as energy convergence value of 1 × 10−5 eV. The external electric field is introduced by planar dipole layer method.42 Given that Fe3O4 belongs to strongly correlated material, the on-site Coulomb interaction parameter U = 4.5 eV and exchange parameter J = 0.89 eV for the Fe atom are set as suggested in the literatures.43,44

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weakly bonded system of C6H6/Fe3O4(001) can be concluded by a small adsorption energy, which is also confirmed by the approximate planar structure of C6H6. Here, the surface and subsurface layers designate the first and second layers of Fe3O4(001) substrate, respectively. Figure 1b shows a top view of atoms sites in the first two layers. For simplicity, FeA and FeB are denoted by A and B, respectively. In order to study the electric field (Eex) effects on C6H6 adsorption, the perpendicular interlayer distances between the first and second layers (d1), the second and third layers (d2), and molecule−surface distance (dC‑M) are plotted as a function of the Eex after lattice relaxations, which is shown in Figure 2a− c. Clearly, there are little fluctuations with varied Eex on the interlayer distances of d1 and d2, revealing that the properties of a deeper Fe3O4 layers are almost undisturbed by Eex. Likewise, the dC‑M is also unchanged when the strength of Eex varies from −0.4 to +0.4 V/nm. As Eex reaches ±0.6 V/nm, a remarkable increase appears in dC‑M. Therefore, the interaction between C6H6 and Fe3O4 will be affected until the field is large enough. The magnetic response of C6H6-adsorbed Fe3O4(001) surface to Eex will be investigated in detail. In a metallic ferromagnet, the applied electric field can induce an accumulation of screening charge near the surface, where the net accumulation of charge is found to be spin-dependent due to the exchange interactions.47,48 Thus, the induced magnetic variation is confined near the surface layer. Table 1 summarizes the calculated atomic magnetic moments under different Eex. When Eex is applied, the magnetic moments of subsurface FeA atoms are nearly unchanged. However, the magnetic moments of partial FeB atoms decrease in the surface layer, where the largest reduction is 1.17 eV from 4.15 to 2.98 μB as Eex increases to ±0.6 V/nm. Most notably, at Eex from −0.4 to +0.4 V/nm, the magnetic moments of the FeB atoms binding directly to the C atoms (B2 and B3) almost do not change, but those of other FeB (B1 and B4) atoms decrease dramatically. These significant modulation on the magnetic moments suggest a magnetoelectric effect in C6H6/Fe3O4(001), which is confined to the surface layer. Furthermore, the induced magnetoelectric effect also can be manifested as a modulation in the spin density. Parts b−g of Figure 3 display the visualized spatial spin density distribution (SDD) of C6H6/Fe3O4(001) in fields ranging from −0.6 to +0.6 V/nm. First, it is seen that the spin coupling between FeA and FeB is antiferromagnetic, in line with the properties of

RESULTS AND DISCUSSION

The relaxed structure of bulk Fe3O4 is shown in Figure 1a. The optimized lattice constant is 8.48 Å, which is well consistent with the experimental value of 8.40 Å.45 As for Fe3O4(001) plane, both FeA and FeBO terminations are considered. In the previous results, the FeBO termination is more energetically favorable,34,46 so only the FeBO termination is considered here. The adsorption configurations include one C atom, C−C bond, and central of six C atoms of C6H6 on the FeB atom of FeBO terminations, which can constitute three high-symmetry adsorption models of top, bridge and center, respectively. The calculated results show that the top model is the most preferable configuration with adsorption energy of −0.82 eV. DFT-D2 method shows a vdW contribution of 0.5 eV, demonstrating the importance of including this term in the standard DFT energy. In what follows, the simulation will be focused on this geometric configuration. The equilibrium structures of top and side views are shown in Figure 1c,d. The B

DOI: 10.1021/acs.jpcc.7b00617 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 2. Optimized perpendicular distances between (a) the first and second layers (d1), (b) the second and third layers (d2), and (c) C6H6 and Fe3O4(001) surface (dC‑M) and (d) the calculated work function (W) of C6H6/Fe3O4(001) as a function of applied external electric field.

Table 1. Calculated Magnetic Moments (Mom) (μB) and Charges (Chg) (e) of Surface O, FeB Atoms, and Subsurface FeA Atoms under Different External Electric Fieldsa −0.6 V/nm OA OB B1 B2 B3 B4 A1 A2 a

−0.4 V/nm

−0.2 V/nm

0 V/nm

0.2 V/nm

0.4 V/nm

0.6 V/nm

Mom

Chg

Mom

Chg

Mom

Chg

Mom

Chg

Mom

Chg

Mom

Chg

Mom

Chg

−0.05 0.20 4.11 4.13 2.98 3.10 −4.07 −4.06

7.10 7.03 − − − − − −

−0.04 0.16 3.00 4.15 4.15 3.09 −4.06 −4.06

7.10 7.03 − − − − − −

−0.03 0.27 3.69 4.11 4.14 2.99 −4.05 −4.05

7.10 7.06 − − − − − −

0.06 0.39 4.13 4.15 4.15 4.13 −4.04 −4.04

7.10 7.11 − − − − − −

−0.02 0.28 3.71 4.13 4.15 3.00 −4.04 −4.05

7.10 7.05 − − − − − −

−0.02 0.26 3.00 4.14 4.13 3.70 −4.05 −4.04

7.11 7.06 − − − − − −

−0.09 0.16 4.11 3.70 3.00 2.98 −4.07 −4.06

7.10 7.03 − − − − − −

Here, OA (OB) atoms represent the O atom with (without) FeA in the vicinity. And the results of OA and OB given are the average value.

Figure 3. Change of the spatial spin density distribution of C6H6/Fe3O4(001) with external electric field Eex = (a) 0, (b) +0.2, (c) −0.2, (d) +0.4, (e) −0.4, (f) +0.6, and (g) −0.6 V/nm. Yellow and blue represent two different spin orientations. The value of the isosurface is 0.1 e/Å3.

the surface layer still maintain Fe3+ states with high-spin electronic configurations of t32g↑e2g↑,32 which can be visualized as the spherical characteristics in spatial SDD50 (Figure 3a). However, the spherical symmetry of SDD for partial surface FeB

Fe3O4. Small yellow spheres represent oxygen atoms. Previous studies44,49 have reported that the surface layer of clean Fe3O4(001) contains exclusively Fe3+. The properties of Fe3O4 get little impact after adsorption of C6H6. So, the FeB atoms in C

DOI: 10.1021/acs.jpcc.7b00617 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C atoms is obviously broken due to the influence of Eex (circled by red lines in Figure 3b−g). In order to better describe it, the surface FeB atoms (including B1, B2, B3, and B4) can be partitioned into two groups: group1 including B1 and B4, group2 including B2 and B3, by considering whether they are bonded to the C atoms or not. At Eex ranging from −0.4 V/nm to 0.4 V/nm, the spatial SDD preserves the spherical shape in group2, implying a high-spin state. However, a significant deviation from the sphere appears in group1. As Eex reaches ±0.6 V/nm, in addition to the atoms in group2, the atoms in group1 are observed to change in SDD. The atoms that change in SDD precisely correspond to those of magnetic moments decreased, which indicates an induced alteration in the 3d electronic states. Several mechanisms have been proposed to explain the induced modification on the surface magnetism in the ferromagnet-molecule structure, including the local p−d orbital hybridization,6,13,18,51 the variation of the hole concentration in the films based on the modified p−d Zener model,17 the molecule-induced surface reconstruction,20 and molecular configuration switch.52 Herewith, the different magnetic response to varied Eex on C6H6-adsorbed Fe3O4 surface will be demonstrated from the induced spin-dependent screening effect. Figure 4 exhibits the calculated differential charge density

the averaged value which may include an offset between the positive and negative Δρ. Although the accumulated charge at Eex = −0.2 V/nm is diminished in magnitude, it just happens in the x−y plane, which is consistent with the slightly increased dC‑M (Figure 2c). Along the z direction, no movements appear. Moreover, the electronic structure of adsorbed C6H6 is almost unchanged at the field of −0.2 V/nm, compared to the situation without Eex (not shown here). It is different at Eex= ± 0.6 V/nm, as shown in the inset of Figure 4c,f. The accumulated charge slightly shifts toward molecular direction. Such small movement is still responsible for the affected C−Fe bonding strength, which is also evidenced by the largest increase in dC‑M (Figure 2c). Therefore, the change of the interfacial bonding strength makes the FeB underneath C atom involve in the surface dipoles formation to screen Eex, which causes the electronic density redistribution and magnetic modification. In addition, Eex also has an influence on the work function (W) of C6H6/Fe3O4(001) system, as shown in Figure 2d. The W is one of the critical surface parameters that determines the minimum energy required to remove an electron from the material. The W in the DFT-slab framework is defined as W = V(∞ − EF), where V(∞) is the electrostatic potential in the center of the vacuum of the slab and EF is the Fermi energy. The totally reduced W mainly results from the Eex-induced surface dipoles. Besides, the magnetic moment of surface O atoms is also sensitive to Eex. According to O atoms on the surface layer with and without subsurface FeA atom in the vicinity, two O sites of OA and OB are defined, respectively, as shown in Figure 1b. For the FeBO termination, the Jahn−Teller distortion influences the electronic and magnetic properties of clean surface atoms.34 As a result, a non-negligible magnetic moment of approximately 0.39 μB is induced in OB atoms. For OA atoms, the average magnetic moment is only 0.06 μB. Both of them almost do not change after adsorption of C6H6. However, the sign of magnetic moments of OA atoms are reversed due to the influence of Eex, but do not change much in magnitude, as shown in Table 1. In practice, it is mainly related to the adjacent subsurface FeA atoms whose magnetic moments align antiparallel to FeB atoms. The OB magnetic moments obviously decrease, which is correlated with the influence of surface FeB atoms. More details will be discussed in the following text. Figure 5 depicts the orbital-projected density of states (DOS) of changed FeB d orbitals in the surface layer of Fe3O4(001) under the Eex of 0 (top), −0.6 (middle) and +0.6 V/nm (bottom). On the basis of the crystal field theory, the 5fold d levels split into triply degenerate t2g (dxy, dxz, and dyz) and doubly degenerate eg (dx2−y2 and dz2) states. Furthermore, the net (spin) magnetization is defined as the integration of difference between the spin-up and spin-down DOS from the bottom of the valence band to EF.53,54 Thus, a variation of the spin-up or spin-down electronic states near EF may cause a modification on the magnetism.53 For Fe3O4, the magnetism of FeB atoms stems from the contribution of spin-up DOS. Because of the influence of Eex, a significant additional occupied FeB-t2g state in the spin-down channel near EF appears, accompanied by the reduction of intensity in the spin-up channel (Figure 5b,c). Meanwhile, the spin-up dx2−y2 state just below EF shifts toward higher energies and becomes unoccupied, i.e., the spin-up eg state has an electronic depletion. According to Bader’s charge analysis scheme,55 the distribution of charge on the neighboring O atoms is totally decreased. That is to say, the intrasurface electron transfer occurs from Fe-eg to

Figure 4. External electric field change of the charge density difference Δρ along the z coordinate at Eex = (a) +0.2, (b) +0.4, (c) +0.6, (d) −0.2, (e) −0.4, and (f) −0.6 V/nm, which is integrated over the x and y direction and normalized to the Fe3O4(001) surface. The red lines indicate the case of Eex = 0 V/nm. z = 0 represents the position of the bottom Fe3O4 layer and starting point of the horizontal axis is the subsurface layer. The inset in part a represents visualized Δρ without Eex at the interface, in which yellow and blue correspond to charge accumulation and depletion, respectively. The insets in parts b and c represent detailed information from the circled contents.

Δρ of C6H6/Fe3O4(001) at different Eex, which is defined as Δρ = ρC6H6/Fe3O4(001) − ρC6H6 − ρFe3O4(001), where ρC6H6/Fe3O4(001), ρC6H6, and ρFe3O4(001) are the charge density of the complete system, isolated C6H6 and isolated Fe3O4(001), respectively. At Eex = 0 (red lines), a net charge accumulation appears around 9.8 eV between C6H6 and Fe3O4, indicating that a stable C−Fe bonds forms between FeB in group 2 and C atoms, which is visualized in the inset of Figure 4a. Because of the influence of Eex, the dipoles form on the surface layer of Fe3O4 due to screening effect, which causes the spatial redistribution of electronic density. In particular, when Eex changes from −0.4 to +0.4 V/ nm, the accumulated charge between C6H6 and Fe3O4 almost does not change, indicating an unaffected C−Fe bond. So, the FeB atoms in group 2 do not involve in charge redistribution. It should be point out that the charge density presented here is D

DOI: 10.1021/acs.jpcc.7b00617 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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atoms are decreased. The hybridization occurs not only in 0.6 V/nm, but also in −0.6 V/nm. This induces a transfer of about 0.2 electrons from OB atoms to the surface FeB atom (Table 1), which is related to the appearance of additional spin-down FeB t2g states just below EF. However, there is no overlap of the electronic states between C6H6 and the d band of Fe in this energy range. So the C6H6 does not directly participate in the variation of orbital occupation but does influence the amplitude of the magnetism on Fe3O4(001) surface by influencing the number of changed surface FeB atoms, which is dependent on the strength of Eex. On the other hand, we also calculate the electric field effect on the magnetism of clean Fe3O4(001) surface. The result shows that the electronic structures and magnetic moments of FeB atoms in the surface layer almost do not change, which emphasizes the role of C6H6 molecule adsorbed on the Fe3O4(001) surface. Furthermore, it is worth mentioning that the different effects may be induced when the C6H6 molecule is placed perpendicular to the Fe3O4(001) surface, since the planar and perpendicular positions of C6H6 adsorbed on the Fe(100) surface displayed different interaction.57 Detailed discussion of comparing two cases will be a further step forward to the understanding of such molecule− surface system. Calculations performed for C6H6-adsorbed Fe3O4(001) surface qualitatively explicate the mechanism of local magnetic variation under Eex, as summarized in Figure 7a by a schematic

Figure 5. Orbital-projected density of states (DOS) of the FeB (a−c) t2g and (d−f) eg states for spin-up and spin-down in the surface layer with Eex=-0.6 V/nm (middle panels), 0.6 V/nm (bottom panels) and without Eex (top panels), respectively. The Fermi level is located at 0 eV.

the spin-down t2g state, with the electronic configuration from t32g↑e2g↑ to t32g↑t12g↓e1g↑ characteristics. Hence the net spin is diminished, which is visualized as a change in the shape of spatial SDD. Similar phenomenon was also reported for Fe3O4(100) surface, which is depicted as the spin flip.56 It is interesting to note that, whether positive or negative Eex, the mechanism of magnetic reduction of FeB atoms is stems from the variation of orbital occupation near EF, which is realized through the orbital hybridization between FeB d and OB p orbitals. Further verification can be derived from Figure 6, in

Figure 7. (a) Schematic spin-resolved DOS of changed surface FeB of Fe3O4 with and without Eex. The magnetic moment of FeB is reduced in magnitude. The increased additional FeB-t2g states come from the charge transfer of FeB-eg and OB p orbitals. (b) Electrically assisted magnetic storage device using C6H6/Fe3O4 system.

model. It can be argued that the reduced magnetism of FeB atoms is a consequence of both electronic depletion of spin-up eg orbital and additional occupation of spin-down t2g state. The results may be expected to provide more potential for developing novel electrically assisted magnetic storage device, in which the storing binary information on “0” and “1” states at the different magnetic system of C6H6/Fe3O4(001) can be defined by changes in the magnitude of magnetism through Eex (Figure 7b). By means of the surface magneto-optic Kerr effect,58 according to the different rotation of the polarization plane of linearly polarized incident ray with reflection ray which is sensitive to the magnetism of probing region, it is promising for the identification of “0” and “1” signals. This novel design is useful for creating advanced magnetic storage devices based on the molecule−surface system. However, for the better application in the practice, further evaluation of the effect of Eex on the magnetism of molecule-adsorbed surface, such as

Figure 6. Orbital-projected DOS for OB (a−c) and OA (d−f) 2p states in the surface layer with Eex = −0.6 (middle panels) and +0.6 V/nm (bottom panels) and without Eex (top panels), respectively. The Fermi level is located at 0 eV.

which the orbital-projected DOS of O p orbitals is plotted with Eex of 0, +0.6, and −0.6 V/nm. Clearly, no significant changes are observed in OA p states (right panels) around EF compared with the case of OB atom (left panels), indicating a relatively weak hybridization between OA p states and Fe d states despite consideration of Eex. As for the electronic structure of OB atom, the degeneracy of px and py orbitals of OB atom is lifted under the influence of Eex. Just below EF, the spin-up px state shifts toward higher energy and across EF, hybridized with Fe eg orbitals. A similar overlap can be seen between OB py states and Fe t2g orbitals near EF. As a result, the magnetic moments of OB E

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calculation of the perpendicular magnetic anisotropy, is required.

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CONCLUSION In summarize, the magnetic properties of Fe3O4(001) surface can be effectively tailored through adsorbing C6H6 molecule with applied Eex. On the basis of the weakly bonded system of C6H6/Fe3O4(001), a reduction in the magnetic moments of partial FeB atoms occurs in the surface layer due to Eex. Meanwhile, the spherical symmetry of spatial SDD is broken, resulting in the reduction of spin states. Owing to the spindependent screening effect, applied Eex induces a formation of surface dipoles which leads to the charge redistribution and reduction of work function. Furthermore, the Eex redistributes orbital occupation between t2g and eg states near EF via FeB d and OB p orbitals hybridization, accompanied by the spin flip. As a result, the magnetism is reduced in the surface layer of C6H6-adsorbed Fe3O4(001), which is irrelevant to the direction of Eex. Interestingly, the FeB atom that is directly bonded to C atom is not disturbed when electric field is changed from −0.4 to +0.4 V/nm. It is varied until the field is increased to ±0.6 V/ nm. The results improve the further understanding of weakly interactive interface and provide a new strategy of electrically modulated surface magnetism through organic molecule, which is very crucial for molecular spintronics as well as technological applications in advanced magnetic and electronic devices.

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AUTHOR INFORMATION

Corresponding Author

*(W.M.) E-mail: [email protected].. ORCID

Wenbo Mi: 0000-0002-9108-9930 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (51671142, U1632152 and 51661145026), the Key Project of Natural Science Foundation of Tianjin (16JCZDJC37300), and the Program for New Century Excellent Talents in University (NCET-13-0409). It is also supported by the High Performance Computing Center of Tianjin University, China.

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